Add test case

test/behavior.zig

@@ -71,6 +71,7 @@ test {
         _ = @import("behavior/bugs/7047.zig");
         _ = @import("behavior/bugs/7003.zig");
         _ = @import("behavior/bugs/7250.zig");
+        _ = @import("behavior/bugs/9584.zig");
         _ = @import("behavior/bugs/394.zig");
         _ = @import("behavior/bugs/421.zig");
         _ = @import("behavior/bugs/529.zig");

test/behavior/bugs/9584.zig

@@ -0,0 +1,60 @@
+const std = @import("std");
+
+const A = packed struct {
+    a: bool,
+    b: bool,
+    c: bool,
+    d: bool,
+
+    e: bool,
+    f: bool,
+    g: bool,
+    h: bool,
+};
+
+const X = union {
+    x: A,
+    y: u64,
+};
+
+pub fn a(
+    x0: i32,
+    x1: i32,
+    x2: i32,
+    x3: i32,
+    x4: i32,
+    flag_a: bool,
+    flag_b: bool,
+) !void {
+    _ = x0;
+    _ = x1;
+    _ = x2;
+    _ = x3;
+    _ = x4;
+    _ = flag_a;
+    // With this bug present, `flag_b` would actually contain the value 17.
+    // Note: this bug only presents itself on debug mode.
+    try std.testing.expect(@ptrCast(*const u8, &flag_b).* == 1);
+}
+
+pub fn b(x: *X) !void {
+    try a(0, 1, 2, 3, 4, x.x.a, x.x.b);
+}
+
+test "bug 9584" {
+    var flags = A{
+        .a = false,
+        .b = true,
+        .c = false,
+        .d = false,
+
+        .e = false,
+        .f = true,
+        .g = false,
+        .h = false,
+    };
+    var x = X{
+        .x = flags,
+    };
+    try b(&x);
+}